Aluminum foil is generally used as an electrode material for aluminum electrolytic capacitors. The surface area of an aluminum foil can usually be increased by performing an etching treatment to form etching pits. The etched surface of the electrode material is then anodized to form an oxide film thereon, which functions as a dielectric. By etching the aluminum foil and applying one of various voltages to the surface thereof to match the voltage that is to be used, an anodic oxide film can be formed, thus enabling various aluminum anodes (foils) to be produced for electrolytic capacitors that are suited to specific applications.
However, etching treatments require the use of an aqueous hydrochloric acid solution that contains sulfuric acid, phosphoric acid, nitric acid, etc., in hydrochloric acid. Hydrochloric acid has a considerable environmental impact, and its disposal is also a burden from the viewpoint of the production process and economically.
Therefore, the development of a novel method for increasing the surface area of an aluminum foil, which does not require etching, has been in demand. For example, Patent Literature 1 proposes a method that increases the surface area by adhering aluminum fine powder onto the surface of the aluminum foil by a vapor deposition method and then sintering. Patent Literature 2 proposes a method that increases the surface area by stacking aluminum particles while maintaining a space between each particle and sintering the particles. It was confirmed that this method makes it possible to obtain a surface area greater than the area of pits formed by etching.
The present inventors tried producing an electrode material comprising a sintered body on an aluminum foil substrate according to the methods disclosed in these documents and found that its bending strength was lower than that of conventional electrode materials obtained by etching. Therefore, when an electrode material comprising a sintered body formed therein is wound to form a capacitor element, the electrode material may be damaged. This problem becomes more notable when fine aluminum particles are used for increasing the capacity. In particular, the sintered body after anodizing (with a chemical conversion treatment) exhibited a remarkable reduction in bending strength. In some cases, the bending number (=the number of bends capable of withstanding breaking) in the bending test was 0. If the bending number is 0, the material will not endure processing in an actual chemical conversion line, thus lowering the mass production capability of the electrode material.